1. Field of the Invention
The present invention relates to a phase shifting device for use in a wireless communication apparatus including a mobile communication terminal.
2. Description of the Background Art
In a 90-degree phase shifter used in a modulator-demodulator for orthogonal modulation signals, in order to obtain proper orthogonal signals, a 90-degree phase shift and equal amplitude need to be achieved with a high degree of accuracy.
As a 90-degree phase shifter having the simplest configuration, there is shown in FIG. 9 a phase shifter of a first conventional art. In the 90-degree phase shifter, the phase of an input signal is subjected to a +45-degree phase shift rotation using a circuit having a high-pass filter configuration composed of a resistor R11 and a capacitor C11. In addition, the phase of an input signal is subjected to a −45-degree phase shift rotation using a circuit having a low-pas filter configuration composed of a resistor R12 and a capacitor C12. Then, the signal having been subjected to a +45-degree phase shift rotation and the signal having been subjected to a −45-degree phase shift rotation are outputted from the respective circuits, whereby orthogonal signals whose phases are different by 90 degrees from each other are generated.
In the aforementioned conventional art 90-degree phase shifter, due to variation in value between elements or the influence of parasitic components in the integrated circuit, errors may occur in 90-degree phase shift and amplitude.
As a 90-degree phase shifting device which allows for the output of proper orthogonal signals by incorporating a correction circuit for correcting the phase error and amplitude error, there exists a phase correction circuit as shown in, for example, patent document 1, etc.
A 90-degree phase shifting device described in patent document 1 is shown in FIG. 10. The 90-degree phase shifting device has the following configuration. Specifically, the 90-degree phase shifting device uses a variable phase shifter 111 composed of variable resistors VR1 and VR2 which are capable of changing resistance values by a control signal, instead of using the resistors R11 and R12 of the CR phase shifter of FIG. 9. Two outputs from the variable phase shifter 111 are inputted to a 90-degree phase comparator 100, and a phase shift comparison result signal obtained by the phase comparator 100, which serves as a phase shift control signal 121, is then fed back to the variable phase shifter 111. By this, the resistance values of the variable resistors VR1 and VR2 are adjusted by the phase shift control signal 121 such that the phase shift between the two outputs from the variable phase shifter 111 is 90 degrees in a stable state.
Note that although FIG. 10 shows the variable phase shifter 111 using the variable resistors VR1 and VR2 instead of the resistors 11 land R12, it is also possible to use a variable phase shifter using variable capacitors instead of the capacitors C11 and C12. In this case, the capacitance values of the variable capacitors are adjusted by a phase shift control signal 121.
The variable resistor may be composed of, for example, a FET, as shown in FIG. 11A. The variable capacitor may be composed of, for example, a diode, as shown in FIG. 11B.
Patent document 1: Japanese Laid-Open Patent Publication No. 8-18397
With the configuration of the conventional art shown in FIG. 10, however, it is not possible to make the frequency of output signals and the frequency of an input signal different from each other. In addition, there is a need to derive, from the variable phase shifter, output signals having the same amount of phase shift rotation as a phase shift determined by the phase comparator. Since there is a limit to the amount of phase shift rotation that can be created with a variable phase shifter having a simple configuration, such as a CR variable phase shifter, it is difficult to output, with a simple configuration, signals having a large amount of phase shift rotation.
Moreover, in the configuration of the conventional art shown in FIG. 10, there are fluctuations in voltage between the input and output of the variable phase shifter 111. Thus, if a variable resistor or a variable capacitor is connected in series between the input and output terminals of the variable phase shifter 111, the resistance value of the variable resistor or the capacitance value of the variable capacitor changes in accordance with the change in potential, causing unstable operation of the variable phase shifter. Accordingly, signals having an exact 90-degree phase shift cannot be generated.